1. Field of the Invention
The present invention relates to a spark plug employed as an ignition device in an internal combustion engine.
2. Related Art
The reduction of fuel consumption by internal combustion engines used in automobiles and other motor vehicles has been a high priority globally due to environmental concerns as well as global-resource concerns. To address these concerns, measures for gasoline engines in particular have been accomplished concerning the implementation of lean burning of fuel and the like through high compression and a lean air-fuel mixture. Because of these measures, however, the discharge voltage of the spark plug has tended to become extremely high.
Furthermore, a DLI ("distributorless ignition") system, which, as the name implies, does not use a distributor, a unit ignition system, and a diode distribution ignition system have been suggested as possible ignition systems. In the DLI system and the diode distribution system, simultaneous discharge to two cylinders occurs with one ignition coil, and so one spark plug functions so that the center electrode side thereof has a high-potential discharge ("positive polarity"), and the other spark plug is such that the center electrode side has a low-potential discharge ("negative polarity"). An ignition system according to conventional methods that use a distributor is a negative polarity system. Here, the discharge voltage required for the positive- polarity becomes extremely high with respect to the discharge voltage required for the negative polarity.
Accordingly, reduction of the discharge voltage has been addressed with extreme urgency, and Japanese Patent Application Laid-open No. 51-91435 and Japanese Examined Patent Publication No. 52-15739 have proposed techniques for reducing such discharge voltage. Fundamentally, each of these documents discloses a spark plug provided with two spark areas, consisting of a first spark gap and a second spark gap formed of a center electrode, a ground electrode bent so as to oppose the tip surface of the center electrode, and another bent ground electrode having a tip surface which opposes a side surface of the center electrode. Efforts are made to reduce the discharge voltage by means of a combined discharge which includes discharge at the first spark gap and discharge at the second.
However, because flame propagation is better the closer a spark area is to the center of the combustion chamber, i.e., the more the spark area protrudes from the metal housing end surface, the higher the torque generated by the engine, but because two spark areas exist in known devices, fluctuations in torque are generated, thus causing fluctuations in engine speed. In short, assiduous implementation has not been carried out due to worsening ignition performance.